1. Field of the Invention
The present invention concerns a method for phase-dependent or phase-sensitive generation of a magnetic resonance image of an examination subject in which multiple coils are used to acquire magnetic resonance image data sets, and a magnetic resonance system to implement the method.
2. Description of the Prior Art
In many applications, conventional magnetic resonance imaging systems use an acquisition system with multiple coils. Different techniques are used in order to obtain a final image. For example, a quadratic summation technique—known as a “Sum of Squares” (SOS) technique—is used. This technique achieves good magnitude images if the signal-to-noise ratio (SNR) is high enough. In measurements with low signal-to-noise ratio, the result is not optimal. An additional disadvantage of this sum of squares technique is that the phase information is lost.
The phase information can be preserved if, for example, a (weighted) linear combination (LC) is used. Alternatively, a method with an adaptive coil combination—what is known as an Adaptive Coil Combine (ACC)—can handle this problem. The resulting image (which comprises phase information) is complex, and the image presentation is optimal even for images with low signal-to-noise ratio. The common problem of these two phase-dependent image generation methods (LC and ACC) is that phase singularities can occur under specific circumstances. Various special solutions in the prior art are known in order to avoid this, as described, for example, in “Reconstruction of Phase Imaging for GRAPPA based Susceptibility Weighted Imaging (SWI)” by Witoszynskyj, Herrmann and Reichenbach in Proceedings of International Society for Magnetic Resonance in Medicine, Vol. 16, Toronto, Canada, 2008, and “A General Method for Generating Multichannel Phase Images without using a Body Coil” by Schäfer and Turner for the ISMRM Workshop on High Field Systems and Applications, Rome, Italy, 15 Oct. 2008. Such methods known in the art have many limitations, however, that makes these methods generally unusable. The effects of this problem depend on the particular application. For example, artifacts with “dark points”—known as “dark point artifacts”—can occur given use of ACC in some applications.